The evolution of optical microscopes has a long history, and the applications of the optical microscopes have extended beyond research and physicians's practice. In a conventional optical microscope, a single objective lens is used to focus on one location of a specimen carried by a microscopic slide and to acquire an image of the location. Dimensions of the image depend on the magnification and the numerical aperture of the objective lens. The image is viewed through an ocular lens or acquired through a camera. Then the specimen is moved and the same image-taking process is repeated at a new location. However, such process is slow for any application that requires a complete view of the specimen. Hence, there is a demand for scanning a large area of the specimen in a minimal amount of time.
One application in which high speed scanning is needed is to scan microscopical preparations for rare cells, e.g. fetal red blood cells in maternal blood during pregnancy, in prenatal diagnosis of genetic abnormalities. Given the fact that methods that can be used to increase the concentration of such cells in the maternal peripheral blood increase the risk of losing some of the cells, it is advantageous to avoid any increase of the concentration. The average frequency of finding fetal red cells in the maternal peripheral blood is one such cell per one milliliter of blood. In order to identify at least 10 cells, which are needed for an accurate prenatal diagnosis of trisomy 21 (Down syndrome), one needs to scan at least 15 milliliters of blood. A microscopical blood smear carries 50.about.70 micro liter of blood, and thus it needs 17.about.20 microscopical blood smears to carry one milliliter of blood. In that regard, about 240 microscopical blood smears are used for 15 milliliters of blood.
A similar need exists in the detection of cancer cells in the peripheral circulation of a patient. The detection of such cells at an early stage can help in a timely prognosis of the disease and increase the efficiency of treatment.
Another application where any kind of enrichment is very difficult, if not impossible, is the detection of specific microorganisms in natural waters. For example, plankton microorganisms in fresh or marine waters are mainly recognized on the basis of their morphological characteristics. Because the majority of species of such microorganisms are still unknown, recording their presence in a particular sample relies mainly on the description of their morphological features. It is often quite difficult to isolate such cells, not to mention further culture them to identify all stages of their biological cycle. It is of the utmost importance for both research and applied purposes (biological monitoring or sanitary monitoring) to have the capacity to scan microscopical preparations of such samples in order to record the presence of these microorganisms.